It's after I have to invent the Lazarus Vector...

Electric buck when you're close, and telekinesis to throw his attacks back at him.

I know you get the reward with 200 ADAM and other goodies after rescuing 3 little sisters. And there are 21 little sisters. So given that 21/3 = 7, does that mean you get the 200 ADAM reward 7 times? Because I've noticed there only appears to be 2 plasmid rewards and 4 tonic rewards associated with these gifts, which only adds up to 6. I'm mainly concerned because I want to conserve enough ADAM to be able to afford all the tonics, but also want to pick up as many Health/EVE upgrades and Tonic/Plasmid slots along the way as needed so I don't have to backtrack to those machines later.

Here is the full list of the 53 Tonics:

Physical: 19 Tonics

BloodLust - U-Invent machine / Inventable after Arcadia
Booze Hound - U-Invent MAchine / Inventable after Arcadia
Eve Link - Sold at Gatherer's Garden machines
Eve Link 2 - Farmer's Market / On the ground near U-Invent & vending machines.
Extra Nutrition - Sold at Gatherer's Garden machines.
Extra Nutrition 2 - Storage Basement of the mall area in Fort Frolic. Covered by metal grate if Cohen dies. Also purchasable.
Extra Nutrition 3 - Research Spider splicers
Hacker's Delight - Medical Pavilion / Eternal Flame / Cremate the body near the camera.
Hacker's Delight 2 - U-Invent machine / Inventable after Arcadia
Hacker's Delight 3 - Point Prometheus / Same room as the Big Daddy Helmet.
Medical Expert - Neptune's Bounty / In the center of the muddy section of the Lower Wharf.
Medical Expert 2 - Fort Frolic / Awarded by an alive Cohen or bought in Hephestus
Medical Expert 3 - Apollo Square / Second floor / Opposite end from 2nd dose of lot 192.
Natural Camoflouge - Research the Houdini Splicers
Scrounger - Research Leadhead splicers
Security Evasion - Arcadia / In the center of a group of splicers, near an Invent machine.
Security Evasion 2 - Hephaestus / Heat Monitoring / Across from electricuted ghost scene, sitting on desk
Sportboost - Research Thuggish splicers
Sportboost 2 - Research Thuggish Splicers

Engineering: 16 Tonics

Alarm Expert - Fort Frolic / Theater / Top Left balcony sitting on the ledge
Alarm Expert 2 - Point Prometheus / Optimized Eugenics / on filing cabinet
Clever Inventor - Suchong's Apartment / Beside the tape recorder
Focused Hacker - Neptune's Bounty / In the area right before fighting Peach Wilkins.
Focused Hacker 2 - Apollo Square / In the Slums near Bathysphere / Atlas's Headquarters
Hacking Expert - Arcadia / Sitting on the first desk in Langford Labs
Hacking Expert 2 - Available at the Point Prometheus Gatherer's Garden
Prolific Inventor - Awarded for saving Little Sisters / Last Tonic reward
Safecracker - Awarded for saving Little Sisters / First Tonic reward
Safecracker 2 - Point Prometheus / Little Wonders / In back room with Gene Bank
Security Expert - Medical Pavilion / On a small chest in the morgue.
Security Expert 2 - Research the flying Security Bot
Shorten Alarms - Neptune's bounty / Near "Saw Masha Today" recording or buy it at a Gatherer's Garden machine in Arcadia.
Shorten Alarms 2 - Hephaestus / On the floor right before setting off the EMP.
Speedy Hacker - Medical Pavilion / On the desk in the dentist's lab.
Speedy Hacker 2 -Available at the Point Prometheus Gatherer's Garden

Combat: 18 Tonics

Armored Shell - Can be purchased at the very first Gatherer's Garden machine.
Armored Shell 2 - Awarded for saving Little Sisters / Second Tonic Reward
Damage Research - Hephaestus / Across from the EMP bomb on a table
Damage Research 2 - Point Prometheus / Upstairs / Center of Library on desk
Electric Flesh - Fort Frolic / The Projector Room in the Theater / On bookshelf
Electric Flesh 2 - Olympic Heights / Fontaine's apartment / In the office on the desk
Frozen Field - Fort Frolic / Awarded for killing the Cohen victim in the freezer
Frozen Field 2 - Hephaestus - Room off entrance to Ryan's office - on desk
Human Inferno - Arcadia Gatherer's Garden machine / 20 Adam
Human Inferno 2 - Purchase at Gatherer's Garden machine in Point Prometheus
Photographer's Eye - Purchase at Gatherer's Garden machine
Photographer's Eye 2 - Research the Rosie Big Daddy.
Static Discarge - Medicial Pavilion / In the hallway after fighting Steinman.
Static Discharge 2 - Research Leadhead splicers
Wrench Jockey - Medical Pavilion / Seen from a window. In a back room, reached by airshaft.
Wrench Jockey 2 - Research Bouncer Big Daddy
Wrench Lurker - Neptune's Bounty / Sitting on the ledge near the plasmid machine.
Wrench Lurker 2 - Available at the Point Prometheus Gatherer's Garden

Like Enrage starts off as 60 but it's only 45 at Fort Frolic. And either Cyclone Trap 1 or Insect Swarm 1 is only 5 ADAM at Point Prometheus, I can't remember which... probably because they figure by that point in the game it's a pretty useless plasmid.

when the splicers begin their siege on the lab, after you put the Lazarus formula into the machine in the office.

I barely used Insect Swarm at all, and I tried to use Cyclone Trap once, on a Big Daddy. I think it was during the part where you can't control your plasmids, and it popped up so I decided to try it, but then I realized that it doesn't work on BDs.

Areas:

Medical Pavilion
Neptune's Bounty
Arcadia
Fort Frolic
Hephaestus
Apollo Square
Point Prometheus

You are a little more than halfway through.