1) yes , both the tasco and celestron models you quoted are
bird-jones reflector , google search the keyword.
They are identified when the focal length stated is much longer than the physical tube length of the newtonian reflector scope. Example 900mm focal length when the telescope is physically 500mm long.
It is due to spherical curvature main mirror and the additional lens at the bottom of the focuser where you put the eyepiece in.
In simple words , it is a limited scope. You should not spend too much money to upgrade it's accessories, the money is better to accumulate and get a better telescope.
Small cheap eyepieces $10 to $30 (ebay plastic aspheric $15) are fine though at this focal ratio.
2) The restriction of bird-jones design is the manufacturing cost allocated to the lens element built-in the focuser.
It cost much more money to produce a small lens that can correct for spherical aberration from the short focal ratio spherical curvature mirror.
To squeeze into the mass-market budget end , lots of sacrifices are made and problems left unsolved. These "problems" or aberrations cannot be easily solved by changing eyepieces.
There is very small difference in production cost (not final pricetag) between spherical and parabolic curvature mirrors at these small sizes as current generation of automatic polish machines can work with parabolic curvature at this size (said to be only 10 minute more machine time) , but consumers (urban living?) prefer a compact looking scope with big numbers in specifications.
The short tube length reduce vibration at a given magnification, allowing the manufacturer to pair it with a smaller mounting that cost less in materials.
Smaller cardboard box size is also cost savings in transportation and retail shelf space.
These are my speculation on why these design of scope are not phased out from production yet.
3) Yes 1.25" interchangeable , no need to follow same brand.
4) Yes, the image would be exciting and impressive for a newbie after getting the EQ mount aligned onto the target. Especially the Moon when it is half-full.
Familiarize with it during the daytime.
http://www.ebay.com.sg/itm/Newest-1-25- ... 1438.l2649
get the 10mm and 23mm , ignore the bad 4mm