Fortigate LAN ports and Fortiswitch VLANs

I must be missing something.

I just got a nice FG101E and a Fortiswitch 424D FPOE.

I ended up with the 101E because the smaller 81E was out of stock so now I have this fun Fortiswitch with 24-ports and a nice Fortigate with 15-ports after the Fortilink connection.

Question:

I've got the Fortiswitch configured with VLANs but I've got 15-ports on my FGT101E that I'd like to use.

Should I not be able to select port-1 through Port-15 and assign them as a switch and use them with the 24-ports on the Fortiswitch 424D so that I have like 39-ports?

Thanks