To my knowledge:

1)E'real- It's very difficult to pull this off in SoloQ. I'd say best case scenario is if you have a corki so that you both could have a very high midgame powerspike with the triforce buildpath and take down the outer turrets so build a large enough lead to make a lategame irrelevant. It's good for poke/siege comps, but sub-optimal if there's not much of a frontline to guard you and the botlane AD.

Pick when: Above example. Non-teamfighting comps (Almost never happens in soloQ).

2)Galio: He's the anti-ap mid, pretty good overall, but his ult's easy to interrupt. His waveclearing is an issue (1 Q doesnt kill casters) and is heavily reliant on the flash ult to make an impact. Better off sticking with Cho.

Pick when: Enemy is extremely ap heavy (at least when both solo lanes are ap)

3)Kog: Has the same issues as ad Kog: Immobile. His ap ratios and waveclear are awesome, but he's a weaker Xerath more or less. Like Xerath, don't pick him into assasins....ever.

Pick when: Teamfight/poke comp involving heavy disengage (Janna,Lulu)

Syndra: Her early game bully status took a hit with the Q nerf, but she's still pretty much a dangerous pick in the right hands. Gets outclassed by current fotm mid solely due to mobility. I will disagree on the thought of Karthus being a better champ (mainly due to depending on player champion proficiency)

Pick when: Skirmish comps with good pick potential, not good against high mobility as you said

Veigar: It seems mages with a lack of mobility is a recurring theme in your pool as well as an emphasis on disengage/re-engage-centric mages in general. Veigar is no exception, however he's a lot harder for me to think about picking. If anything, it's against an ap heavy comp (Like Galio) with a lack of a frontline (Think lulu/rumble).

Overall, your current champion pool is fairly good for the current meta whereas your considered selections would have issues against tanks. The lack of assassins does hurt your ability to help the team by winning lane and applying pressure to the enemy mid due to your (I assume) farm-heavy style, but don't let me or anyone else deter you from what you want to play. Best of luck:)