Congrats on the new to you car, that's a nice one.
Model specific details for pre-2001 model year Cadillacs are not listed in the LATCH manual but I can help you out any way to some degree.
A '99 model pre-dates when lower LATCH anchors were mandatory factory equipment starting with the 2003 model year, so it is very unlikely to have lower LATCH anchors, and lower LATCH anchors cannot be retrofitted. The Seville didn't have lower LATCH anchors in the 2001 model year, only top tether anchors, so that pretty much rules out the chance of your '99 model having any lower LATCH anchors. But that's not the end of the world at all because you would just use the vehicle seatbelt to install your carseats. Lower LATCH anchor installs are technically no safer than a seatbelt installation, or vice versa, really, as long as you can achieve a proper installation with less than 1 inch of movement at the carseat's belt path.
I do recommend getting the car retrofitted with top tether anchors for use along with the vehicle seatbelt for installing forward facing harnessed carseats down the road a ways once the kiddos max out the rear facing capacity of the Uptown and/or whatever rear facing convertible you opt to use after the Companion is outgrown. :twocents: Top tethering serves to reduce head excursion during a crash and it's recommended for forward facing harnessed seats in vehicles with an appropriate top tether anchor. If your car is not already factory equipped with top tether anchors, and it probably isn't because it predates when tether anchors became mandatory factory equipment in the 2000 model year, then a GM dealership should be able to look up the tether anchor part number for your Caddy to retrofit it with top tether anchors. You'll need to refer the GM dealership service department to GM Technical Service Bulletin Number 99-09-40-004a dated April 12, 2005 regarding tether anchor retrofits. The first tether anchor retrofit would be free of charge according to that bulletin, and any additional ones (probably up to 3 total depending on the exact specs for your car) would be out of pocket for you and might possibly be a do it yourself task with a torque wrench to save on the labor charges beyond the first free tether anchor.