Re: SSLVPN TO IPSECVPN TO CISCO
If I'm understanding this correctly, you want SSLVPN users that connect to your Fortigate to be able to get across the IPSec tunnel from that Fortigate to a Cisco endpoint? If that's your requirement, then yes this is possible. I have this configured on all of our office Fortigates although the IPSec tunnels terminate on VyOS routers - still the same deal, however. In my scenario, our SSLVPN users are using tunnel mode connections and our IPSec tunnels are interface mode.
Without knowing much about what your configuration looks like, the one thing that comes to mind that you may be missing is you have to include your SSLVPN pool addresses in your tunnel selectors at both ends of the tunnel and also any rules that you may have in place.