Yeah, if you know your tank is too small, better to get rid of it now than later. Trying to get rid of a larger pleco can be difficult.
And ancistrus are commonly known as bristlenose plecos, in case you were wondering.
I've got one Ancistrus, one Bulldog, and one Clown Pleco. One in each of my main tanks. All of which are plecos that only get to be around 4" or so.
I highly recommend the Ancistrus over the others, simply because I never see the others. They add to my tanks bio-load, and that's about all they do. Besides, the bristles on ancistrus are just awesome, lol
Many plecos prefer to be alone, and will chase others away when they see them. I'm not sure if this is the case with bulldogs as I've only ever kept one pleco to a tank, but I'd do a little reading on it before assuming it wants a similar species friend.