I would always go second, since you want to get 2 or more cards in the same column, to di this, set Spell cards first, then summon Mekk-knights - you can get 2 or more cards in the same column to Special Summon Mekk-Knights. I strongly recommend to check out not just mine, but other people's Mekk-Knight decks, also Invoked Mekk-Knights are still great decks to build. The reason I have Yellow Star is to get rid of backrow. Link first, then Xyz. I recommend the following Xyz monsters if you don't have any of the Extra Deck Monsters:
- Number 38: Hope Harbringer
- Number 5: Doom Chimera Dragon
- Number 23: Dark Knight of the Underworld
- Galaxy-Eyes Photon Lord
- Galaxy-Eyes Cipher Dragon
Replacement for Link monsters you don't have:
- Borrelguard
- Hip Hoshingen
- Knightmare Phoenix
- Underclock Taker
NOTE: Feel free to replace anything in this deck with other things you think will work better for you. This is just what I think would be best for this deck.