I find some confirmation of that information in (the short) Appendix C of
https://www.nxp.com/docs/en/reference-manual/M68000PRM.pdf. From that it does appear that S1, S2, and S3 records can be intermixed in a single SRecord file.
However, it also speaks of S9, S8, and S7 termination records for "blocks" of the corresponding records types. Since addresses are optional in termination records, it does seem that it is possible to create a run of S1 records, terminate them with an S9 record which has no address, and then begin another run of, say, S2 records to finally be followed by an S8 record with a starting execution address. So only the final termination record needs to have a starting execution address (or somehow make provision for a default provided by the OS).
But every block of S1, S2 or S3 records has to have its own terminator record, if I'm reading this correctly. In any case, no matter how I read it, the familiar 'S19' record file, with only S1 data records and a single S9 terminator record, is valid. So are 'S28' and 'S37' files.
On the other hand, so would a 'S1S9S2S8S1S9' file be, I think.
And on the other other hand...whatever works