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in order to synchro, you need one tuner monster, no more no less, and depending on the synchro requirements listed on the monster, 1 or more other monsters, sometimes of certain types or attributes (like how XX Saber Gatoms needs a tuner and a at least one non-tuner warrior). the combined level of the monsters used for the synchro summon must equal the synchro monsters level, so it's harder than a ritual summon, and may require some time to pull off. the plus side, is you dont need any special cards like polymerization or a ritual spell, you can use any tuner unless the synchro monster states otherwise (like how black wing synchros need a blacwing tuner)

yeah they have to equal the level exactly. it's annoying, but most tuners are low level with good effects (like krebons whose level 2 and can avoid dying by paying 800 LP, or Junk Junk Synchron whose level three and gives you a free level 2 or lower monster from your graveyard when normal summoned (thats 4 or 5 stars right there, just need 1 or 2 more for a goyo guardian))

Some good combos are:
Armeggedon Knight + Plaguepsreader zombie
Junk Synchron + Charcoal Inpachi (good defense, one star)
Dark Grepher + Sirocco + Vayu + Gale (two synchros right there)

Old post, don't care. To correct this statement:

Link.

Monsters required for Synchro Summon are all listed on the card, such as with Fusion Monsters. The process of Synchro Summoning is all the same, sending the requirements from the Field to the Graveyard, unless specified to be in a different manner in the text of the Tuner Monster(i.e., Eccentric Boy allows a Synchro Summon with a Monster(s) from the hand).

Axean was wrong in the summoning of XX-Saber Gottoms, specifically. It does not require a Non-Tuner Warrior-Type Monster, only EARTH Monsters. Also, it specifies that a Tuner is needed, but not if the rest must all be Non-Tuners. Indeed, XX-Saber Gottoms can be Synchro Summoned with multiple Tuner Monsters in the mix, so long as they follow the requirement to be EARTH Attribute.