That's a very generous offer from your friend. You'd be very hard pressed to find used parts for a 4000 gal for just 2000 dollars. Anyway as far as your future kitten goes. A 55 gal will do for a year. A 125 would be a more ideal starter tank. Although I usually use a series of starter tanks liking to be able to observe every stage of developement. After a year your kitten should be hovering around the 14" - 20" mark deending on it's growth rate. A varied diet is good. I usually start mine off on bloodworms. Worked very well for me. If the rtc is extremely small you can actually use an eyedropper to suck up the thawed bloodworms mixed with water and literally bottle feed your rtc so to speak. Gradually I add chunks of shrimp or prawn, small ammounts of fruits(if he'll take them), fish fillet(any kind), mussels, crabmeat(this is their natural food), insects, live worms, ect. I've even fed snakes before, but that was maybe kind of a silly thing to do, but he ate them. Remove any uneaten foods from the water within an hour of offering them. Try to offer foods after lights out if he won't eat during the day. Keep your water very pristine especially when they are small or they might develope gill curl or a bladder infection. I'd recommend PH 6.8-7.2, water 82-82 F, and the rest should be common sense. Do very frequent water changes. I use to do them at least 50% twice per week, and that seemed to keep things under control no matter how much I fed. You can use a submerssible titanium heater it will just need to be encased in something so he doesn't burn himself. I use to keep mine under some big rocks. Worked like a charm. Unlikely that your cat will jump(rare for rtc), but just ensure that it's covered. A sump is a type of filter that allows you to put massive bacteria loving biological material inside to suite your filtration needs. The water generally is syphoned out the back of the aquarium down to the sump where it goes through a prefilter to remove heavy debris, then on to biochambers, and can also include an air diffuser, heater, ect. Keeps your tank kinda clean by keeping the junk out lol. Look in the DIY section under setups and filtration for more info. You can feed your cat daily when he's small. After he hits the 10" mark or so feed him every other day. try not to overfeed as he will puke and kill himself. Just feed till the belly is rounded, but doesn't look like a bomb about to go off. If he does puke, clean the tank with a 75% waterchange, and monitor the params. Do more water changes as needed. Hint-water changes are key to keeping a healthy rtc!!! I think everyone's had the idea to grow their own feeders, problem is that you won't be able to grow enough to keep him fed. After he gets to about 14" he could eat your entire stock in one sitting. to support an adult rtc that weighs 50 lbs he would need to eat 10% of his body weight nightly. that's 5 lbs a night. How many mollies would that take? You would need to have a massive pond with probably 10-20k pairs of mollies breeding on a regular basis to keep a rtc fed. You would spend more on food for them then just buying some shrimp and fish fillets at the store for your rtc. Live fish are always a very bad idea unless you know they are not sick for a fact! Trust me you don't want a sick rtc. YOu will lose many nights of sleep trying to keep up on water changes and trying to get him to eat again if he does get sick.
When the rtc outgrows the pleco you will start to notice damage to the pleco at first and then when it seems all aggression has stopped you will wake up one morning with the pleco sticking out of the mouth of your rtc. Since plecos are so boney it's hard to digest and he will likely try to puke him out possibly killing your rtc. Don't get a filter for the 55 gal, instead use the 55 gal as a sump filter for a 125 or 250 gal starter tank. It will just outgrow a 55 in a matter of months is all. You'll regret not having a bigger tank. You'd be better to start in a 75 gal if you can't afford a 125 off the bat. 75 gives you a much nicer starting foot print than a 55. IMO a 55 is no better than a 40, 30, 20, or 10 gal unless you have small ornamental fish. Yes they do and will grow that fast unless you starve them, and that's just cruel!
sorry not trying to turn you off of rtc, just want you to know the truth. People on other sites would just ridicule you until you went away, but I believe in providing real facts to help you make a good decision on what to do. I think you should ask your friend a little more about what he would put into a $2000, 4000 gal tank. Just doesn't seem feesible no matter how you look at it. $3000 would still be a stretch. $4000 would be amazingly cheap and unheard of. Yes that's minus labor. I have built a 4000 gal almost to completion before so I know. If this doesn't turn you off of getting the fish then go for it, and feel free to ask more questions. That's what this site is all about. Helping others keep their fish well.
This is normal and it will turn red when he's 8-14".