Medium sized pleco's can hold there own. One that gets to at least 6". Are your cichlids full grown already? As long as your pleco has a hollow log type hiding place to call they're own they should be fine. I don't have a ton of experience with pleco's so I couldn't tell you exactly what kind to get. Also maybe a large snail (one the cichlids can't fit in there mouth would help). You could also just lessen your light time a bit. Thats what I do with my tank.
55g - 3" Long-Finned Albino Oscar
10g - Heavily Planted, M GBR, 3 Otos, 6 Schooling Rummy's