Ok, I've figured out how to do what I need. Thanks for the advise, I used both find_clades and depths

tree = Phylo.read('tree.nwk', 'newick')    

depth_threshold = 0.008
groups = {}
full_clade = []
cnt = 0
for key in tree.depths():
    if key in full_clade:
        # put it in group
        groups[str(cnt)] += [key]
        continue
    if tree.depths()[key] < depth_threshold:
        if str(key) != 'Clade':
            cnt += 1
            groups[str(cnt)] = [key]
        continue
    if str(key) != 'Clade':
        # separate seq to group alone
        cnt += 1
        groups[str(cnt)] = [key]
        continue
    if tree.depths()[key] >= depth_threshold:
        # save everything in this clade
        full_clade = key.find_clades()
        cnt += 1
        groups[str(cnt)] = []
        continue

print('Number of clades: ', len(groups))
print('clade\tn_seq')
for i in groups:
    print(i, '\t', len(groups[i]))