I'm a bit confused, you said you were getting a 27 gallon but this has changed to 36 gallon? Which one is it?
I think what you mean is a plecostomus. The one you mentioned is likely to be Hypostomus plecostomus and will eventually grow far too large for a 36 gallon. Did you know they can get to over 2 feet in length?
I see many things wrong with your current set-up, and I'm not having a go (so I hope it doesn't sound like it) but I think you need to know some facts about your fish.
Silver dollars grow to be around 6inches across. They'll do fine in a 36 gallon but a 15 gallon is way to small.
Silver-tipped sharks, also known as bala sharks, are a similar situation but grow to be up to 16inches long. Again, too big for 15 gallons or even 36 gallons.
Goldfish grow up to a foot long, and it's a bad idea to mix tropicals (all your other fish) with coldwater fish (your goldfish) for a long period of time.
My advice is this: if you want to start over, research your fish thoroughly before you buy them, I'm the same as any other person, when you go to buy one fish it turns into buying 5 fish and often we make bad choices in what we buy. I would seriously consider puttin gthe silver dollars and silver-tipped shark in your friends 90gallon where there'll be much happier.
Please read up on cycling a tank properly. You may already know this but it's always worth covering. Cycling isn't simply letting the tank run empty for a couple of weeks, it's the gradual build-up of colonies of beneficial bacteria in your filter and on rocks and stones and aquarium decorations. What this bacteria does is convert the harmful substances in fishes waste (ammonia) into less harmful substances and then prevents many problems when you come to add fish.
Also, a good rule of thumb for people new to fishkeeping is "one inch of fully-grown fish per gallon", so for example neon tetras grow to an inch long, you can safely house 15 in a 15 gallon and then it'd be fully stocked. This only applies to small, slim-bodied fish (the majority of fish for sale in the hobby), and not fish like goldfish. Goldfish are generally reccommended for 20 gallons for a single fish, adding 10 gallons for each additional fish.
If you want a pleco or "sucker fish", the one's I'd go for are the ones that stay small. Look out for Bristlenose Plecos, Rubberlip Plecos and Bulldog Plecos, which all stay around 6 inches long.
I hope I haven't over-loaded you with information and I hope this all helps, but as I've said to others on here it's an expensive hobby at the best of times and sometimes when we make bad decisions or bad judgement we can be literally flushing money down the toilet!
Please keep us updated on your decisions.