yeah i would try and get a bigger tank, and if you really want a pleco, stay away from commons they get WAY to big for a 55g and are not very efficient algae eaters once larger anyways, you can get a couple bristlenose plecos and they should be fine but i would put them in first, main thing to keep away algae though is just keeping the water clean, if your water is clean and has no extra nutrients for the algae to use then it wont grow in the first place. also Jag will grow WAY to big for that tank, especially if its a male, if its a female you might be able to get away with having both of them in a 75g tank but 55g is still way to small. also you would need to upgrade sooner than 6mnths, within 6mnths your JD will be about 6in and your jag will be about 8in and if it doesnt feel it has enough room will have no trouble making more room by killing of the JD.
as for a divider, i wouldnt bother with it if your putting them in at the same time, just keep a close eye out for trouble and be ready to divide if need be, but for that tank you would be much better off with a single JD and some dithers or a single GT or something along those lines
WELCOME TO MFK, you definitely came to the right place to get help, seems like you have a good interest in these awesome cichlids and that you actually care about keeping them correctly, so just keep reading and asking on here and you should know all you need to know soon enough GOOD LUCK