I have ordered 9 chicks to arrive in June. 4 bantum and 5 standord hens.
Very few hatcheries sex bantam chicks. I'm not sure where you are getting them but if they are chicks it is possible you will get an extra boy or two anyway.
I am considering adding a cream legbar roo. Will he be too large for my bantum birds
Bantams come in different sizes. For Silkies, in the USA the SOP is for a 32 ounce hen, in Australia the SOP calls for a 20 ounce hen. Not sure what the UK Silkie SOP calls for. Not all Silkies follow the SOP for sure and this is for just one breed. Different bantam breeds call for different sizes. Cream Legbar roosters can weigh 7-1/2 pounds though many are smaller.
When chickens mate, the hen squats. That gets her chest on the ground so the rooster's weight goes through her body and not just through her legs. That's how a smaller hen can mate with a larger rooster without getting hurt. The more difference in weight the more the risk, but many people keep full sized fowl roosters with bantam hens without problems. I don't know if he will be OK with your bantams or not.
and will this be enough hens for 2 roos?
That magical, mythical, mysterious boy-girl ratio where everything between boys and girls is true bliss. I know, you read about that all the time on this forum. From what I've seen that doesn't work in the real world. Two boys will fight over 25 girls as fast as they will fight over one. Some girls can be over-mated or barebacked whether the ratio is 1 to 1 or 1 to 20. You just don't get guarantees with living animals, almost anything can happen. Your risks are much higher during puberty than once they all mature but the risks even then are a long way from zero.
To me, one critical factor is how much room they have. Your odds of the boys getting along are much better the closer you are to free range as opposed to the BYC minimums of 4 square feet in the coop and 10 per chickens outside. Another big factor is the individual personalities of the individuals, both boys and girls. The girls have a part to play in this too.
I do not know how the differences in sizes will work out. I don't know how two boys with your girls will work out. The only way you can know for sure is to try it. I would observe and be ready to react quickly if you need to separate some.