They shouldn't... I have black neons that don't get sucked up a filter larger than anything you could put on a 5g. If you have a 5-10g filter, they'll be fine.
For tetras, I don't think it particularly matters on the sex (I'm not even sure how you tell in stores because they generally aren't fully grown). If you get platies, you either want all males or females, or a 1 male:2 female ratio, so a single female doesn't constantly get pestered. If you have females, you'll probably end up with babies at some point.