Hi Do The Dew or whatever are you,
Some like to complicate things, I like to simplify. Every question you ask or are going to ask has been answered countless times on these forums, all you have to do is read. Now please allow me to baffle you with my bullstuff, I can't dazzle you with my brilliance.
"I am using a 800mhz disk style antenna and it has a 75ohm coax connector on it."
There is no such thing as a 75 ohm connector.
"If I run 75ohm coax to my 800mhz scanner how will reception be?"
I haven't a clue.
"Its about a 35' run."
Not far so it shouldn't be tired at the finish.
"Will I see a big loss in signal compared to 50ohm."
About 25 ohms worth.
"The antenna is similar to the "Discadoo" antenna but made by another company."
The name on the label is irrelevant, yours is Rob, mine is Warren, we are both men (or I hope you are).
OK, now my ascerbic wit is out of the way. For that short a run any cable besides RG-58 or 59 will do nicely. Considering loss as we all do RG-58 is the cable of choice for mobile installations because it's thin and easy to work with and it's only about 10' long, otherwise it's for HF only. For runs of about 50' or so most common cables like RG-8AU polyfoam are just fine and for about 100' you start thinking Belden 9913 or Times Microwave LMR series. More than that and it's semi-rigid air dielectric or hard line, the longer the run the more loss, the "better" the cable the less loss, things tend to balance out.
OK, now that I've totally confused you (I love to compete in that department) go do your homework. Download a coax chart (manufacturer's specs) and you'll see more accurately than we can describe in words exactly how cable length vs. frequency affects signal loss. Then research connectors and adapters, you'll see they have no particular characteristic impedance nor do they have any measurable signal loss. Thus armed you're prepared to do battle against the misinformation so common on this Battlefield Earth.
Pardon me, I also like to steal titles, this one from L. Ron Hubbard. If you think Dienetics stinks on ice you haven't read his childish science fiction, even John Travolta couldn't save the movie. (;->)
Oh, almost forgot!
"Those websites are great tools."
You've only begun to scratch the surface, wait until you start reading some comprehensive study material on RF theory. Then your name will be up there in lights just like mine and Jerry Lewis (The Nutty Professor) but I don't have buck teeth.