I use a SP GT2 with an Xcelorin 6.5T motor, saddles and a Savox 1251 servo (low profile).
With that setup a 20T pinion works well for me but does depend on the track.
The SP esc will fit behind a low profile servo (although v.tight), but most drivers i've seen are running their esc at the back.
Hope this info is useful
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