1. After you have made a custom planet, how can you get them to appear in the game? I have noted that even maps with random mix do not EVER choose the new planet types.
Most maps are made non-randomly, so new planets you add won't show up in them. To make them show up in random maps, you'll have to add them to the galaxyscenariodef file. They're split up into groupings which the game uses (some official maps do allow for variance so you may get your new one, just not guaranteed). I'll post a small bit:
planetTypeCount 26
planetType
designName "WeightedNonIce"
designStringId "IDSGalaxyScenarioTypePlanetWeightedNonIce"
orbitBodyTypeCount 6
orbitBodyType "Terran"
orbitBodyType "Desert"
orbitBodyType "Volcanic"
orbitBodyType "Asteroid"
orbitBodyType "Asteroid"
orbitBodyType "DeadAsteroid"
The planetType is the "grouping", so whenever you add the WeightedNonIce group (easiest to see in the in-game generator), it can pick from any of those planets. The orbitBodyTypes are defined a little later in the file:
orbitBodyType
typeName "Desert"
entityDefName "PlanetDesert"
defaultTemplateName "Template:DefaultStart_Desert"
So you can see the link. In the planetType it uses "Desert", which is linked to the PlanetDesert.entity file by the orbitBodyType definition in the second paste.
2. How do you alter the max capital ship level? I have altered gameplay constants and tried extending it, but to no avail.
As far as I know, 10 is the max and you can't go higher. Lower might work, but I'm not sure.