Well, if Dell does not recommend it, then I suppose you should not do it. However, if you are will to maybe risk paying return shipping you could try the 16GB stick of RAM, but that is something you must decide for yourself to see if it actually works despite what Dell states.
As for upgrading the SSD, it really depends on how much storage capacity you need. If 256GB is adequate enough for you, then there is no real reason to upgrade now. Waiting also means that you can eventually pay less as holiday sales appear and more competition and higher capacity SSDs appear on the market driving prices down.