Fine tuning underdark scout...

[Naanomi]

Upcoming campaign is possible underdark heavy (the plane's surface is a near uninhabitable wasteland)

Two questions...

I'm thinking a scout-y type character; svirfneblin Beastmaster ranger; ride that giant spider pet as follows:

8/16/14/14/14/8
Perception, Stealth, Survival, Investigation, Animal Handling//Thief's Tools, Cartographer Tools
Underdark Terrain; Abberation Enemy
ASI: mounted combat, svirfneblin magic, +2 Dex, +2 Dex, ???

Anything I'm missing here? Suggestions on last feat choice? (Alert, observant, dungeon delver, other?)

Second question: we are going to have all the underdark races in the party (Drow, svirfneblin, deuregar)... Suggestions on the race of the fourth? Dark vision seems a must... Half Orc and Tiefling closely mirror deuregar and Drow... Fire Gensai perhaps? Or a warlock or barbarian to get it from class features?

[Specter]

Well, first of all, why INT14? Seems like a complete waste.
If you're going for Beastmaster, I'd say Wood Elf is a lot better than Svirnefblin, but you might be set on the concept. 120' darkvision hardly matters more than the '60 darkvision.

[Naanomi]

Design thoughts:
-I want to ride my spider around on cavern walls, so halfling or gnome... Only gnome gets darkvision; so just choosing between gnome variants, of which svirfneblin seems an obvious winner
-14 INT doesn't do me much good (though I do have investigation), but I'm rocking +2 from race and don't have much need for stats elsewhere... Suggestions on other options for stat placement?

Rest of the party now decided:
-Drow swashbuckler (maybe dipping battlemaster later); Noble
-Deuregar champion; merchant
-V. human Goo/tomelok; hermit