first of all as previously mentioned that is a House rule. non marginable is not the correct term. from a legal point they can be margined at the current 50%. it is marginable but at 100 percent per some house rule.
According to FINRA rule 2520, if you make frequent security trades and certain other .... If the stock closes below $5.00, the maintenance requirement is 100% of .... and above $5.00 Special NASD Notice to Members 99-33 securities position where the stock sells at $5 per share or above, Rule. 2520 requires maintenance margin of $5 per share or 30 percent of the current market . Just a cursory google search, my Compliance Officer is away today, LOL. As I said, however, we "can" give some wiggle room for adequately capitalized traders. We are only concerned when the stock in question takes up all the trader's equity. This leaves no room for normal trading. Don
Seems to me the best qualified to judge the risk are those who run the prop shop. If they are telling you that a concentrated position in a particular security is high risk ... well, they are probably correct. The key has to be making money within reasonable risk parameters; not just making money till the risk catches up to you.