One solution would be to have different maximums for different skills. For example, if Fireball were already maxed out after investing 3 points and Sticky Ball after investing 4 points, then Sticky Fire Ball would become obtainable on level 2.
This of course puts a limit on how far certain skills can be developed. How committed are you to the idea that a skill must be maxed out before it unlocks a merged skill? You could already enable certain skill mergers on earlier levels. So a character with Fireball 3 or Stick Ball 4 would have the options to invest into Sticky Fireball 1, Fireball 4 or Sticky Ball 5.
If you do want to stay committed to the idea that mergers may only happen when a skill is maxed out, want mergers to happen on low levels but also want to give the option to level fireballing to absurdly high levels, then you could solve that by maxing out skills early, but also unlock a more advanced version of a skill when it's maxed out. For example, you could max out Fireball on level 3, and then not just unlock the Stick Fireball, but also unlock a separate skill "Greater Fireball", which is basically just a stronger version of Fireball but mechanically counts as a separate skill.