Not Necessarily, I Solo'd stuff with 50 lvls below the cap skill , so 300~304+ you are good to "SOLO" Stuff but of course
354 + Cap merits = make you an Amazing Machine :D hehe....
you can start as soon as you hit 90 even with those skills, but try a lower tier NM to test out if you can handle it.
when i hit 90 thf my eva was capped for a 90 blu and ninjutsu skill helps as a sub but not 100% needed to get shadows to land. i went out and tried to solo Marvin in la thiene platue due to its high double attack rate it was indeed a chalenge but doable. it got easier the more skills in eva i got and the more i worked on my eva set. so minumin doesnt matter really to start soloing aby NM's as long as you plan to work on your skill and gear at the same time as you solo the lower tier NM's to get practice for the HIgher teir NM's. /dnc is good for nin to help keep you alive and less reliant on meds like thf needs for soloing NM's. and for the sake of other jobs DO NOT MERIT EVA it takes away possible weapon merits from any other melee jobs you have, and you can make up for it by useing an eva set.
Again , It is possible to do SOLO without being CAP , I done it XD (That is how I managed to get CAP)
It depends mostly on what you're trying to solo, although I do think that in all cases where I have solo'd something I could have probably done it regardless of what my ninjutsu skill was. Evasion plays a pivotal role in most rewarding solos, so make sure to cap that. I have 4 merits in evasion because I enjoy ninja more than any other job. There's no reason to forego defensive combat skill merits in order to strengthen other jobs if that's not what you want to do.
For my personal experience, you'll do fine with about 300 ninjutsu and about 290 evasion. But note, anything under the cap for ninjutsu will make your enfeebles suck on the mob. Anything under the cap for evasion, and you'll spend more time rotating shadows than fully damaging the enemy.
Being capped should be stressed, especially evasion. Trust me, you'll rather have DD or Haste gear to make the job easier/faster than having to substitute all your equipment slots for evasion.
Capping your evasion takes a few minutes in abyssea with regen/pdt atmas and a lot of mandies, go do it now. Capping ninjutsu takes a bit longer, but is by no means difficult. You should be able to do it within one evening of spamming nukes/enfeebles on exp mobs.
You should always try to keep your priorities straight. There are very easy things to do in the game that improve your job tremendously (skillups, for instance). There are very hard/time consuming things to do that improve your job less than skillups would (like soloing that NM 20 times for a single piece of gear).
You can solo anything as long as you can keep yourself alive for long enough to make the group of people waiting for you to kill it offer to kill it for you (and likely proc it and offer TH on it).
The trick is to accept their help and not make them sit there for an hour waiting for you, because that's not a good way to make friends.
If you take an hour soloing something as nin, you're doing it wrong. :/