I would prefer 8 or 10 nt UDIs instead. Is this possible?

Yes.

Lexogen UDI 12 nt Sets contain a unique nested feature enabling adjustable index read-out lengths where 12, 10 or 8nt can be read-out.

The UDIs are 12 nucleotides long, however, the superior error correction feature can also be used if only 8 or 10 nucleotides of the index are read out (only if the i5 index was read out in the correct orientation see Figure 1 and also the following FAQ: How do I choose the right Lexogen UDI 12nt Unique Dual Indexing Set?).

When pooling Lexogen UDI libraries with libraries from other vendors, you can select the index length of your choice (e.g., 8, 10, or 12 nucleotides).

NOTE: As error correction with idemux is not possible in this case, the indices will adhere to the standard Illumina error correction procedure. This will result in the same number of lost reads as with any other index type.

Figure | Optional read-out lengths for Lexogen UDI 12 nt Sets.