those orange caps are hit-or-miss. i have a couple that work fine and a couple that have trouble sealing.
for the very beginning of fermentation it should not matter as you will have plenty of co2 escaping, preventing anything else from getting in. you will need to have a better bung when it slows, however, to prevent oxygen or anything else from meandering in there. You can use a drilled stopper. I believe the glass carboys take a # 8.