Connectors for access to four conductors from a 40-wire ribbon cable
First, you generally should not power peripherals (at least beyond a single low current chip) from a pi's 3v3 bus; it is too sensitive to additional and varying loads.  For low current things, use the 5v bus and regulate it yourself; for higher current ones, use an independent connection back to the power supply, or a separate power supply with a common ground, or even isolate both signals and power.

UART signals at reasonable baud rates are in general not very sensitive to impedance match over short distances.

In terms of your connector options, you have many:


You can crimp an additional full-width IDC connector onto an existing ribbon cable.  Use a vise.
You can split out a few conductors and crimp them into a smaller IDC connector.
You can split out a few conductors and individually crimp male or female pins on them.  Note that you can also use these female pins on a male crimp IDC connector, but you likely won't have any good secondary retention so that is best for temporary setups.
If the overall number of connections to be made is small, you can use individual crimp pins inserted into hearders (either Molex or Dupont style; they mate, but the housings and pins are not cross compatible).  You can crimp these yourself, buy them pre-crimped without housings or buy them with single housings and pop those off.
You can solder individual wires to the back of the pi or the other board, 30 gauge silicone insulated wire is great for this as you can do a neat job with the insulation right to the solder joint without it melting.  You can then bundle these, maybe in some heatshrink and strap them at a mounting hole or standoff or similar.
You can use a breakout board having multiple points of access to the signals including an additional connector footprint.  Beware many of these are not reversible as they have the power pins of the pi footprint internally connected, however compact pi-zero sized breakout boards will in general work with the other models, too.
You can make a custom board