I have two Sailfin Pleco's about 3 - 4 inches long. I have had them for about one year. I found out recently they grow very big, and I only have a 50 gallon tank. So, I want to get Bristlenose Pleco's. In the meantime, how long can I keep the Sailfin Pleco's before they get too big for my tank? Also, is it a good or bad idea to get my Bristlenose Pleco's now, or after I re-home the other Pleco's, when they get too big for my tank?

when to rehome them is a hard one. I would suggest that when they are about 20-25cms long they will prob be too large for the tank. I am sure others will know more about this as i have never kept them before. if this size is too large will someone correct me

however, how long it will take them to get to that size is unsure.

If they are still small then i can see no issue with getting the bristlenose now. I would make sure there are lots of places to hide in the tank and i would set up at least two feeding areas to reduce agression at feeding time

Your tank should house them fine for quite a while, and getting bns now to go with them is fine. Generally plecos grow to the size their tank will allow them to be, so they may not actually get giant.
My sister has 2 sailfins as well in a 4 foot tank (48G I think) and both of them are a 20cm or so, they're only with another couple of fish though but they're quite happy in that tank. Far as I know it's taken them 2-3 years to get that big, so if you need to rehouse yours, you've got plenty of time. :P

Generally it depends on the conditions the fish are kept in. In breeding conditions (plenty waterchanges and fresh veg) the fish will grow nicely. But if just kept in a community tank where food is basically what food drops to the bottom and the odd algae disc then yeah they gonna grow slower. The right conditions for the larger growing plecs is about 20cm in the first 12 month but it then slows down a bit. After a further 12 months the fish should be a healthy 30-35cm. Whith gold spot plecos the size won't get much bigger but with commons and sailfins they should at least double that size to be full size.
As for having 2 in a 50G, the first 12 or so months should be fine but after that they will begin to get too big and also much more territorial towards eachother.
Adding bn to the tank now will be fine but when the plecos start to get bigger feeding time becomes more difficult for the smaller bn.
Hope that helps

You may find it easier to rehome the Sailfins while they are still small. LFS may take young fish but are usually reluctant to take larger specimens. Sadly a lot of these fish are bought on a whim and buyers are not told by some LFS how big they can grow, resulting in a lot becomeing unwanted when they grow to big.

keeping them in a tank that is to small for a year or so is ok but after that its very very bad for the fish as they might get stunted witch is VERY bad for the fish and will cause an early death. You see as the fish stunts there body won't grow but all there internal organs will continue to grow to normal size. Causeing pain and early death for the fish.

Silvashadow your statement is very misinformed. I'm sorry but your doing as great disservice to the hobby saying "Generally plecos grow to the size their tank will allow them to be, so they may not actually get giant." I hate being so blunt about this as I used to think the same thing not knowing the truth about stunting til I did more reading and discovered how wrong this statment is. If you had a greatdane you wouldn't keep him in a closet.Even if it were for a few months til he grew up. As it would be cruel to the dog..fish are no different than dogs when "we" choose to keep them as our pets

after a fish is full grown its ok to keep them in a smaller tank IE for breeding ect. thou i still wouldn't myself but many breeders do. As they can't be stunted and will not hurt them.They might not like the smaller quarters it won't harm the fish physically.
there is lots of misinformation on this subject all over fish forums and I have read most of them..If you want to read more about it..find a ichthyology site to read more about there findings not these type forums as they are very helpful, most sites use allot of hearsay and no Scientific proof behind there claims.

please don't take this personal as I just want to inform and not just say your wrong...its important for all us and our fishy friends