Feats are based on Hit Dice, not character level
When you take monster class levels, you're entering a progression that includes both Hit Dice (levels where your HP, and skills, as well as sometimes BAB and saves are increased) and level adjustment (levels where you only get the features, but no statistical bonuses).
For a normal player character, their level is equal to their Hit Dice (they have no LA). For a mind flayer, the 15 levels in the monster class are the combination of 8 Hit Dice and 7 LA.
Thus, a hypothetical mind flayer 1/psion 6 would have 7 Hit Dice. A hypothetical mind flayer 5/psion 2 would have 5 Hit Dice (because two of the mind flayer levels are LA, denoted by the dash in the skills part of the table).
As a note, though, you cannot multiclass out of Savage Species monster classes. Per Savage Species p. 152:
A monster character may not multiclass until it completes the full progression in its monster class. This rule keeps characters from gaining the benefits of a monster’s type and then quickly switching to a standard class.
Personally, I think that much of this time, the rule is somewhat of a mess and lead to underpowered characters. LA is a downside, and largely, monster classes are much weaker than normal PCs (though in some cases it can be problematic in the other direction). Regardless, the answer above would still apply in the cases where, say, someone takes all 15 mind flayer levels before entering psion.