yes... common plecs can get HUGE. best to take him back. there are a few dwarf pl*c varieties that are cute and make for decent algae eaters... i recently got a young rubberlip, and he/she had my glass clean overnight. however, these guys are probably too big for your 5g as well.
amano shrimp (cardinia japonica) are a good bet... they will eat just about anything. and their wacky antics are great when you catch them out and about.
otos are great too... stay fairly small, and some of the cutest fish in the world imo.
and for the sake of your gups... avoid chinese algae eaters and golden algae eaters like the plague. these are the guys that are usually just labelled "algae eaters" at most lfs. it's a bit of a misnomer; they actually prefer giving your fish vicious skin-breaking hickies and sucking off their slime coat to eating algae... a condition that only gets worse as they mature and get huge.
however, you should not be relying on fish to keep algae at bay. there are likely more sinister workings afoot that are causing your algae bloom, whether it be poor light spectrum (is your light source fluorescent? color temp. at least 5000 Kelvin?), excess nutrients in the water (is there anything weird added to your area's tap water?), too much nitrate (perhaps your fish need more frequent water changes?), need for co2 injection, etc.
that being said... i think you're probably at or extremely close to your limit in a 5 gallon tank with six guppies. any more fish added could arguably constitute overcrowding. so perhaps consider a larger setup, or getting more anal about tank maintenance, scrubdowns, etc.