Will my large pleco mind smaller plecos?

Dont even bother uprading to a 55. Just go straight to the 75g. 55 has no added depth and the length is not that much of a help. Addonis grow monster, as in over 2 feet.

a 75g is only a 6" difference than a 55g to be honest. Basically it all depends on what kind of pleco you have. If it's a common pleco they can reach 20" so I suggest a 90g or larger tank if you plan on keeping more than a single pleco. Some plecos can tollerate others being in the tank, and some can't. Pleco's just like any other fish fully depends on each fishes personal temperment on whether or not they can tollerate others in the tank.

i think the amount of aggression will depend on the fish. i currently have 4 bristlenose plecos, 3 clowns (panaque maccus), a royal pleco, and a common pleco altogether in a 150g tanks. they range in size from 1.5 - 10 inches long. they do just fine with each other. when the bristlenose plecos were first introduced back to each other (they were all siblings purchased at the same time, but kept in different tanks for over a year) they did fight and it looked very violent, but none of them had damage once things were settled. before this tank, the common pleco (10 inches) was kept with a very evil rubberlip pleco (3 inches) who would constantly drive the larger fish away from the food.

if you give your plecos some of their own room to make territories, there wont be much aggression. Like andrew said, go for the 75g, as a minimum.

and yes, most larger fish and pet stores do carry tanks above 75g. If you're worried about price, check out craigslist.com for some hot deals on used tanks.

Add to that some plec species show agression only within their own species. Adonis can be a little territorial with other adonis, most people don't carry tankage to hold one adult, much less two. Royals can be very agressive to other royals, etc etc. When you upgrade to 75 or larger, you can try another species of smaller plec, that does'nt resemble the adonis in pattern.