The way I was thinking this would work would be:
My characters are modelled with 4 guys on the base with them. Just ignore that. I guess their base size is a bit larger than it should be.
Tactical squad bases have only regular marines on them. Each squad is 10 man and has a missile launcher. Same weapons everywhere to make things easy to remember.
Devastator squad bases have a missile launcher guy on them. Each squad is 10 man and has 4 missile launchers. Same weapons everywhere to make things easy to remember.
Bikes are 9 man squads but modelled as 6 because it makes more sense to have 1 base represent the minimum squad size of 3 (bases in epic were always 2-3 bikes).
Dreadnoughts are all contemptors with assault cannons as modelled.
Land Speeders all have the same weapons across the whole army to make it easy to remember.