At ten months that Sussex boy is probably old enough to act more like a mature rooster than an immature hormonal driven cockerel. That's to your advantage.
Are the two EE pullets laying yet? Are the two EE and the three silkie chicks integrated yet or have they been kept separate? I'll assume the silkies are bantam. They can be hard to sex. It could help to know if there are any boys in them so you'd know something else to look for.
What do your facilities look like? How big, in feet or meters, are your coops and runs? How are they tied together? Some photos could really help with this, both outside and inside the coops. It is easier to respond if I know what you are working with. Where are you located so we have an idea about your weather?
A good mature rooster should not try to mate the silkies. If they are not laying he probably won't the EE pullets either, but if he does they can handle it. An immature cockerel can be unpredictable but not all mature roosters are good either. If they were young enough a good mature rooster would assume they are his and not likely to harm them. The hens would be more of a danger, even the calm sweet ones. Usually the hens aren't that bad either but they do not think those chicks are theirs. If the EE's are laying they may be accepted into the mature pecking order without drama or they may fight for status in the pecking order. If they are not laying they are basically older chicks and will probably keep their distance from the older ones. I assume the older girls are 10 months, same as the boy, and are laying or have been.
If one or more of the Silkies are boys the Sussex boy may see them as a rival and could be a threat to them, either now or later as they continue to mature. Something to watch for.
When they mate the girls squat. That puts their body on the ground so the rooster's weight goes into the ground through the body instead of the legs. That's nature's way of protecting them from leg injuries. Some people have large roosters with bantam hens and don't have problems, but the more difference in weight between the two the more risk there is. It's not real common but some people do have injuries from that. I can't give you any guarantees of what will happen. It could go either way but probably not a worry until they start laying.
Without knowing what you have to work with and whether the EE's and silkies are integrated it's hard to get too specific. There are some generic things that can help, not sure how any would apply to you. Housing them across wire to get them used to each other can help. Having multiple feeding and watering stations can help. The more room you can give them the better. You can improve the quality of what room you have with clutter. Clutter is giving them places to hide under, behind, and over. You might be able to use the 'safe haven" concept for the silkies, the EE's are too big for that. That's where you provide a safe place for the silkies with openings big enough that they can get through but small enough that the others can't.
If you have a lot of room this might not be that bad. If it is as tight as your smaller number of chickens imply it might be then it could be more challenging. But it is still probably doable.