Best bet is to manually deal with the algae yourself before adding livestock to deal with it. Livestock creates ammonia which will contribute to the algae possibly and not eat the right kind, depending what algae it is.
If its green spot or green glass, use a credit card or algae scraper, take care to mind the seals... don't wanna nick those with a razor blade.
Green water... drain it and refill with clean water.
And find out WHY it has algae. Sounds like it might be an outdoor tank, which is exposed to natural lighting, which won't make it easy to keep algae at bay.
A pleco for a 70 gallon, most larger plecos aren't algae eating fish... honestly I'd go for the common bristlenose pleco (ancistrus cf cirrhosis). Hardy, not overly large, they will eat some soft algaes, and they're not super shy like many plecos can be. A 70 gallon can only house small or medium sized plecos. Basically anything that stays less than 8 inches long. But plecos are awesome, with the right research. Would need a heater as well more than likely.