SFs, in the ideal world, complement your other starting 4 to create a complete unit. i never really found well-rounded SFs to exist very widely, up until high d1, when suddenly there's like 15 750+ SFs every season. d3, d2, low d1 i usually just went with a guard - but i pressed. i think you can generally go either way, get a big, get a guard, it can work. as long as the SF is good at a couple things, like reb, scoring, defense, or bh/pass (probably less important than the other 3), they are going to be alright. try to avoid setting minimums like 30 reb or 40 passing for these players, players are just too imperfect down there, and its much better to get a SF who does 2 things well than one who does 4 things decently.
when you are trying to compete for deep NT runs, that is when you really want to focus on team synergy. the nice thing is, your SF can literally be anyone. if you are lacking per scoring, throwing a per scoring SF in (basically a guy you'd play at SG but with a bit more tolerance for less-than-exceptional spd/bh), can be huge for those teams. if you are pressing, throwing in a strong press defender who can help generate turnovers, can be huge for you. i tended to just treat my 3 as another guard, when i played d2/d3, because i pressed. in man or zone, i'd probably be keen to get some strong rebounding to help me on the possession game, the press can win the possession game against all other sets because the turnover generation is so strong, and the only real way to strike back against that with non-press sets, is to try to dominate on the boards.
anyway, just be flexible and don't be concerned with minimums, its alright for your SF to be absolutely terrible at something a SF would traditionally do in your mind. as long as they bring important strengths you'll be fine. i never really liked SFs who were **** defenders, but they can definitely work, especially for teams outside the top 10 or so.