Yes, RAC expansion/renovation and football indoor practice facility are priorities, but if RU Lax alumni are willing to pony up $20-30 million, there's absolutely NO reason NOT to build a lax-only stadium. It is VERY distracting watching lacrosse at SHI with all of the football markings on the field, and in a cavernous stadium. Take a look at the PSU, Hopkins, and UMich lax-only stadium facilities and tell me it's not 100% better to watch those games.
As for the seating, the lax-only stadium would probably have 5,000 seating capacity (just a guess). Most games wouldn't get that many people, and for the ones that do, you just make sure there is "standing room" capacity around the perimeter of the field.