I am not sure, but I think you are referring to running multiple one-click install scripts, with different PHP requirements, on the one domain. If so, then yes, that could be a problem.
One work-around would be to install the scripts to sub-domains. For example, put your forum on forum.yourdomain.com and your gallery on gallery.yourdomain.com. Remember, under the DreamHost system, sub-domains are treated pretty much the same as full domains and you can specify the PHP version to be used by them, independent of the parent domain.
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)