Hi Huinan, thanks for reaching out.
Regarding your first question on mag uncertainties: there are columns with SNR values, i.e. snr_g, snr_r, etc. To first order the uncertainties on the mag_g, mag_r are thus sigma_mag_g = 1/snr_g, sigma_mag_r = 1/snr_r, etc.
On your second question: the LS team does not compute the radii r25, r50, etc, but you can get something similar in the tables ls_dr8.apflux_n and ls_dr8.apflux_s; there you will find columns like apflux_g_1, apflux_g_2, ..., apflux_g_8, which are (circular) "Aperture fluxes on the co-added images in aperture of radius 0.5, 0.75, 1, 1.5, 2, 3.5, 5, 7 arcseconds".
Hope this is helpful.