I always did it that way with my Lexicon U42S which has pre-amplified line/mic inputs...
That may be the case, but if you read the U42S manual, you'll see it's still not advisable to connect line-level gear to XLR mic inputs. Here's what it says:
Connecting the I•ONIX U42S
Microphones
Plug an XLR cable directly from a microphone into the desired Mic/Inst input on the U42S rear panel.
Line Level Sources
Line level sources include keyboards, drum machines, CD players, and external microphone preamps and effects. Plug a 1/4" TS (unbalanced) or TRS (balanced) cable directly from the line level source into the desired Mic/Inst input on the U42S rear panel.
Specifications
Microphone Inputs: Female XLR Pin 2 Hot
Input Impedance: 3kΩ balanced
Line Inputs: 1/4” TRS balanced or unbalanced
Input Impedance: 20kΩ balanced, 13kΩ unbalanced
I would bet the majority of any other interfaces you find will have XLR sockets for mic-level signals, and 1/4" jack sockets (either balanced or unbalanced) for line-level signals. It's usually ok to plug an unbalanced TS output plug into a balanced TRS input socket - some inputs compensate automatically as they detect the fact that the Ring is shorted to Screen, or there may be a switch as with the Tascam 16x08, but at worst you'll just lose some signal level. If your synths have unbalanced 1/4" outputs, I would use a TS-TS cable.
Don't get confused with 1/4" 'instrument' sockets. The term can be confusing as it doesn't really mean any instrument, e.g. as in a synth. It really means it's a special high-impedance (or Hi-Z) input to match the outputs from sources such as an electric guitar coil pickup or a piezo-electric pickup.