You can always invest anytime on those casinos. Most casinos have a minimum of 0.01btc for investing on their bankroll
I dont really think that 0.01 btc is something that is worth to invest. Sometimes there are thousand btc in one site so 0.01 is not really worth it. You could have better chance of getting profit if you just gamble that instead of throwing it away into investment, you wont get any profit because It is like less than 0.1 % of the bankroll
That's correct its very small investment to make and even if you make profits you will never feel the profits as it will be too low so if I have that amount to invest then I will prefer to directly gamble with that and who knows if I am lucky then I can make really big from it.