Is it OK to mix Colony Wars and Frontiers for a 2 player game?
"Balance" typically refers to each player's relative chances of winning.  There isn't any inerrant balance changes from the expansions because the trade row is shared between the two players.  Even if the trade-row gets full of high-value cards, the game does not favor one player over the other.  Unaffordable trade rows happen even with just the base game (I've seen it happen), and Explorers exist for this very reason.

Balance issues aside, all of the Star Realms expansions have a spread of card costs.  I do not believe any of them significantly increase the chance of an un-affordable starting trade row.